Endoscopes are well known devices for visually inspecting inaccessible places such as human body cavities. Typically, the endoscope comprises an elongated insertion tube with a handle at the proximal end as seen from the operator, and visual inspections means, such as a built in camera, at the distal end of the elongated insertion tube. This convention of distal and proximal, proximal being the end closest to the operator and distal being the end remote from the operator, as used above for the endoscope in general will, where applicable, be adhered to for all parts throughout this description. Electrical wiring for the camera and other electronics such as LED lighting run along the inside of the elongated insertion tube from the handle to the tip at the distal end. Instead of using cameras, endoscopes may also be fibre-optic, in which case the optical fibres run along inside of the elongated insertion tube. Also, a working channel may run along the inside of the insertion tube from the handle to the tip, e.g. allowing liquid to be removed from the body cavity or allowing the insertion of surgical instruments or the like into the body cavity.
Furthermore, in order to be able to maneuver the endoscope inside the body cavity, the distal end of the endoscope may comprise a section with increased flexibility, e.g. an articulated tip part allowing the operator to bend this section. Typically this is done by tensioning or slacking pull-wires in a guide tube also running along the inside of the elongated insertion tube from the articulated tip part to a control mechanism with a control knob in the handle in an arrangement commonly known as a Bowden cable, cf. Bowden's original patent U.S. Pat. No. 609,570.
The pull-wire running along the inside of the guide tube of a Bowden cable normally extends with a predetermined length over either end allowing an operating member to be attached to a free end, in the following referred to as the proximal end, and an operated member to attached to the other free end, in the following referred to as the distal end. When the ends of the guide member are held stationary, movement of the proximal end of the pull-wire with respect to the guide tube is transmitted to the distal end as a corresponding movement of the distal end of the pull-wire with respect to the guide tube, so as to effect a movement of the operated member.
It is a known problem with Bowden cables is that the functioning of the Bowden cable is influenced by changes in the relative lengths of the pull-wire and guide tube with respect to each other. If the relative lengths change, the ends of the guide tube may be still be considered stationary but the starting point of the transmitted motion is changed. This, in turn, means that the starting point of the motion effecting the movement has and also changed, and the movement is not transmitted as desired to the operated member. In practice, the length of the distal free end of the pull-wire must be maintained with respect to the distal end of the guide tube, which can be considered a stationary reference point for the distal end of the pull-wire. Causes for such changes in the relative lengths of the pull-wire and the guide tube could be the pull-wire stretching, e.g. under tension, or the guide tube shortening, e.g. under compression or due to shrinking of the material of the guide tube due to ageing.
In conventional disposable endoscopes, where manufacturing costs have to be kept down, the guide tube is made of a polymer plastic. Polymer plastics are prone to shrinking, as they deform plastically under tension or pressure over time, in particularly if the temperature increased above room temperature, e.g. during transport or storage in a container in the sun.
If, in an endoscope, the relative length of the pull-wire as compared to the guide tube increases, the movement of the control knob is only transferred in part to the operated member, i.e. the articulated tip part, because the tension of the pull-wire changes, and effectively the starting point of the motion moves accordingly. This leads to a smaller maximum deflection of the articulated tip when the control knob is moved to its most extreme position in the handle, i.e. where further motion is limited by abutting the handle.
This problem of reduced deflection of tips of endoscopes has been dealt with in the prior art, e.g. in U.S. Pat. Nos. 5,167,221, 4,203,430.
U.S. Pat. No. 5,167,221 discloses an endoscope having means for tensioning the pull-wires, e.g. before shipping, during use if the pull-wire slacks over time, or for fine adjustment. This is generally achieved by forming the housing of the handle in two housing parts, which may move with respect to each other in a telescopic manner. The proximal ends of the pull-wires are fixed to the control knob of the control mechanism in one of the housing parts, whereas the proximal ends of the guide tubes abut the other housing part. In one embodiment the two housing parts may be moved with respect to each other using a threaded circumferential ring engaging a corresponding thread on one of the housing parts. This arrangement is quite complicated and does not lend itself to disposable endoscopes, where the manufacturing costs have to be kept down.
U.S. Pat. No. 4,203,430 discloses an arrangement where the proximal ends of the pull-wires are terminated in internally threaded adjustment cylinders which engage an externally threaded part of the racks of a rack and pinion control means. By rotating the cylinders the tension of the control wires may be adjusted. Since, however, the racks, pinion, and adjustment cylinders are all located inside the handle housing such an arrangement is unsuitable for a disposable endoscope because the handle housing would need to be taken apart for adjustment prior to use if the guide tube has shrunk during storage.